Closed loop backdrilling system

ABSTRACT

A multilayer circuit board is provided with at least one signal layer, at least one feedback layer, and at least one dielectric layer positioned between the signal layer and the feedback layer. The signal layer is connected to at least one plated hole. The feedback layer has a contact pad, which is positioned adjacent to the plated hole, but is electrically isolated from the plated hole. The contact pad is connected to a measurement unit. The dielectric layer is positioned between the signal layer and the contact pad of the feedback layer. A portion of the plated hole forms a stub portion, which extends a distance away from the signal layer and typically extends a distance away from the contact pad of the feedback layer. To remove the stub portion, a hole is bored or routed into the multilayer circuit board until electrical feedback is received by the measurement unit upon contact of a portion of the boring device with the contact pad. Upon receipt of the electrical feedback by the measurement unit, the boring device is retracted from the hole, and the hole formed by the boring device is filled with an epoxy, or other filler material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No.10/944,583, filed on Sep.17, 2004, which claims priority to the provisional patent applicationidentified by U.S. Ser. No. 60/504,399, filed on Sep. 19, 2003, of whichthe entire content of which is hereby expressly incorporated byreference.

BACKGROUND OF THE INVENTION

Multilayer circuit boards and/or wiring boards are well known in theart. The multilayer circuit boards are formed with a number of signallayers (conductive layers) which are arranged in a predeterminedpattern. The signal layers are insulated from each other by dielectriclayers. Thus, the multilayer circuit board is formed from interleaved(e.g., alternating) signal layers and dielectric layers.

Plated holes or “vias” are formed through the multilayer circuit boardto connect one signal layer to another signal layer. The plated holestypically extend from one main side of the multilayer circuit board toan opposite side of the multilayer circuit board. In some instances, theplated holes or vias include a “stub portion” which extends away fromthe signal layer toward one of the sides of the multilayer circuitboard. In some circumstances, it is desirable to remove the stub portionto enhance the signal to noise ratio in electronic systems that utilizehigh-speed signals. The smaller the stub portion the better the signalquality.

In the past, the stub portion of the plated holes is removed in aprocess referred to as “back drilling”. In the back drilling process,the stub portion of the plated hole is removed by drilling the stubportion to a predetermined depth. However, in practice the thicknessesof the various layers in the multilayer circuit boards are not uniform,and the depths of the signal layers within the multilayer circuit boardsvary, which often changes the amount of the stub portion which needs tobe removed. Drilling too deeply will disconnect the signal layer fromthe plated hole, or leave an unreliable connection; not drilling deepenough decreases the signal to noise ratio.

The invention discussed herein uses a technique which overcomes theproblems associated with the variance in thickness of the layers in themultilayer circuit board.

SUMMARY OF THE INVENTION

In general, the present invention relates to a multilayer circuit board,and a closed loop backdrilling system for backdrilling the multilayercircuit board. The multilayer circuit board is provided with at leastone signal layer, at least one feedback layer, and at least onedielectric layer positioned between the signal layer and the feedbacklayer. The signal layer is connected to at least one plated hole. Thefeedback layer has a contact pad, which is positioned adjacent to theplated hole and is connected to a signal/electrical source, but iselectrically isolated from the plated hole. The dielectric layer ispositioned between the signal layer and the contact pad of the feedbacklayer. A portion of the plated hole forms a stub portion, which extendsa distance away from the signal layer and typically extends a distanceaway from the contact pad of the feedback layer. The feedback layer canbe any layer within the multilayer circuit board, such as a separatenetwork layer as shown in FIG. 2, or a ground layer, signal layer, orpower layer.

To remove the stub portion, a hole is bored or routed into themultilayer circuit board until electrical feedback is received from thefeedback layer of the multilayer circuit board when a certain depth isreached. Typically, the electrical feedback is received from themultilayer circuit board upon contact of a portion of the boring devicewith the feedback layer. Upon receipt of the electrical feedback fromthe multilayer circuit board, the boring device is retracted from thehole, and the hole formed by the boring device can then be filled withan epoxy, or other filler material.

In one preferred embodiment, the feedback layer is only used during themanufacturing process of the multilayer circuit board. In other words,in this embodiment, once the manufacturing process is complete, thefeedback layer is not used to connect any components or signal layers orfor any other electrical type of purpose.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a diagrammatic view of a closed loop backdrilling systemconstructed in accordance with the present invention.

FIG. 2 is a top plan view of one embodiment of a feedback layer formedin a multilayer circuit board wherein the feedback layer forms aseparate network.

FIG. 3 is a cross-sectional view of a portion of a multilayer circuitboard constructed in accordance with the present invention.

FIG. 4 is a top plan view of a contact pad of a feedback layerconstructed in accordance with the present invention.

FIG. 5 is a cross-sectional view of a portion of a multilayer circuitboard constructed in accordance with an alternate embodiment of thepresent invention.

FIG. 6 is a cross-sectional view of the portion of the multilayercircuit board constructed in accordance with an alternate embodiment ofthe present invention.

FIG. 7 is a top plan view of an alternate embodiment of a contact padconstructed in accordance with the present invention.

FIG. 8 is a diagrammatic view of an alternate embodiment of a closedloop backdrilling system constructed in accordance with the presentinvention.

FIG. 9 is a diagrammatic view of yet another alternate embodiment of aclosed loop backdrilling system constructed in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings and in particular to FIG. 1, shown thereinand designated by a reference numeral 10 is a closed loop backdrillingsystem constructed in accordance with the present invention. The closedloop backdrilling system 10 is used for depth drilling an already platedhole 12 formed in a workpiece 14 using electrical feedback from theworkpiece 14 when a certain or predetermined depth is reached. Thefeedback during the drilling process makes it possible to yield muchhigher accuracy independent of the depth of a particular layer withinthe workpiece 14. In one preferred embodiment, the workpiece 14 is amultilayer printed circuit board 16. Although the workpiece 14 will bedescribed herein as the multilayer circuit board 16, it should beunderstood that the workpiece 14 can be any apparatus, or device whereit is desirable to drill to a certain or predetermined depth within theapparatus or device.

The multilayer circuit board 16 is provided with at least one or moresignal layer 20, at least one or more feedback layer 22, and at leastone or more dielectric layer 24. The signal layer 20 is connected to theplated hole 12. The feedback layer 22 has a contact pad 26 (see FIG. 4).The contact pad 26 is spaced a distance away from the signal layer 20and is electrically insulated from the plated hole 12. The dielectriclayer 24 is positioned between the signal layer 20 and the contact pad26 of the feedback layer 22 so as to electrically isolate the contactpad 26 from the signal layer 20. As will be discussed in more detailbelow, the contact pad 26 is contacted during the manufacturing processof the multilayer circuit board 16 to provide electrical feedback.

The multilayer circuit board 16 is also provided with an entry material28, and a second plated hole 30. The entry material 28 extends over afirst surface 32 of the multilayer circuit board 16. Entry materials,such as the entry material 28 are well known in the art. Thus, nofurther comments are deemed necessary regarding the entry material 28 toteach one skilled in the art how to make or use the present invention.The second plated hole 30 is electrically connected to the contact pad26 of the feedback layer 22.

The closed loop backdrilling system 10 is provided with a boring device40. The boring device 40 is provided with a spindle unit 42 supporting acutting device 44, and a servo control unit 46 for the Z-axis motorcontrol. The boring device 40 can be a drill, milling machine or router.

The closed loop backdrilling system 10 is also provided with ameasurement unit 48. The measurement unit 48 communicates with 1) thespindle unit 42 and/or the cutting device 44 of the spindle unit 42, and2 ) the second plated hole 30 for determining the contact or proximitybetween the cutting device 44 and the contact pad 26 of the feedbacklayer 22.

In general, the measurement unit 48 senses a transition of a physicalproperty indicative of contact or close proximity between the cuttingdevice 44 and the contact pad 26. In one preferred embodiment, themeasurement unit 48 includes a capacitive sensor due to the airbearingconstruction in the drill spindle for sensing the capacitance betweenthe cutting device 44 and the contact pad 26. In another preferredembodiment, the measurement unit 48 is a resistance meter for measuringthe resistance between the cutting device 44 and the feedback layer 22.In this instance, the cuffing device 44 is constructed of a conductivematerial, or is plated with a conductive material. The cutting device 44can be constructed of a cutting type material, e.g., a ceramic materialor a metallic substance, such as steel.

When the cutting device 44 contacts the feedback layer 22, theresistance rapidly reduces. A signal indicative of this rapid transitionis transmitted to the servo control unit 46 for controlling the spindleunit 42. When the cutting device 44 reaches the feedback layer 22, thespindle unit 42 retracts the cutting device 44 from the plated hole, ora signal is sent to the servo control unit 46 to cause the servo controlunit 46 to measure further depth drilling beyond the feedback layer 22.The servo control unit 46 can be programmed to cause the cutting device44 to move in a substantially continuous manner during the backdrillingoperation. The servo control unit 46 can also be programmed for otherdrilling operations, such as pulse drilling, e.g., move in-stop, movein-stop, etc, or customizable drilling operations. The advantage ofpulse drilling is that it breaks the burr (e.g., a thin ridge or area ofroughness produced in cutting or shaping metal) while drilling andenhances the backdrilled effect. Although the present invention isdescribed as using the servo control unit 46, it should be understoodthat the present invention can use other types of controllers forcontrolling the movement of the spindle unit 42.

As shown in FIG. 3, the cutting device 44 forms an enlarged portion ofthe hole 12 having a backdrill diameter 50 during the back drillingoperation. The backdrill diameter 50 (or cross-sectional area of theenlarged portion) is greater than a first drill diameter 52 (orcross-sectional area of the plated hole 12) of the plated hole 12 toremove the plating from the stub portion 18 of the plated hole 12. Thus,the cutting device 44 removes the plating of the plated hole 12 duringthe backdrilling operation until the cutting device 44 is retracted. Itshould be understood that the diameter of the cutting device 44 can beany size capable of removing the plating of the plated hole 12. In onepreferred embodiment, the cutting device 44 has a diameter approximately0.35 mm greater than the diameter of the cutting device used to form thefirst drill diameter 52 of the plated hole 12. Preferably the cuttingdevice 44 forms a round hole. However, it should also be understood thatthe cutting device 44 can be translated or paired with another cuttingdevice so as to form a substantially square hole, elliptical hole or thelike.

One preferred embodiment of the contact pad 26 is shown in FIG. 4. Thecontact pad 26 can have the general form of a “doughnut” forming a hole60 therein. The contact pad 26 can also be provided with other shapes solong as a portion of the contact pad 26 is positioned adjacent to theplated hole 12 to be backdrilled. For example, shown in FIG. 7 is analternate embodiment of a contact pad 26 a formed as a portion of aground layer within the multilayer circuit board 16. The contact pad 26a forms one or more hole 60 a, i.e., a clearance hole or antipad, withthe plated hole 12 extending through the clearance the hole 60 a. Thecontact pads 26 and 26 a are desirably constructed of an electricallyconductive material. The size of the holes 60 and 60 a can vary widely,but are preferably greater than the first diameter 52, yet less than thediameter 50 so that the contact pads 26 and 26 a do not come intoelectrical contact with the plated hole 12. In this regard, the contactpads 26 and 26 a extend adjacent to the plated hole 12, but do notcontact the plated hole 12.

Although the contact pads 26 and 26 a have been shown and described withregard to FIGS. 4 and 7 as having a doughnut design or rectangulardesign, it should be understood that the contact pads 26 and 26 a canhave any geometric, nongeometric or asymmetrical shape so long as thecontact pads 26 and 26 a are electrically insulated from the plated hole12 and capable of contacting the cutting device 44 during the backdrilling operation.

As shown in FIG. 2, it should be understood that, in one preferredembodiment, for every back drilling depth a pattern (on a specificlayer) is used which is electrically connected to the second plated hole30. The second plated hole may also be referred to herein as a “commonpad”. The common pad 30 is electrically connected to a plurality ofcontact pads 26 with each of the contact pads 26 being provided adjacentto a signal layer 20 and a plated hole 12. In other words, in apreferred embodiment the feedback layer 22 forms a predetermined patternhaving a plurality (two or more) of contact pads 26 with each of thecontact pads 26 being electrically connected to the common pad 30 via apattern of conductors 72. In one preferred embodiment, the feedbacklayer 22 is not contacted or connected to any of the signals or signallayers in the printed circuit board 16. It should be understood that thefeedback layer 22 is not limited to being implemented as a separatenetwork as shown in FIG. 2. In this regard, the feedback layer 22 can beformed by any conductive layer within the multilayer printed circuitboard 16 capable of providing an electrical signal to the contact pads26 or 26 a, such as a signal layer, a power layer or a ground planelayer.

The measurement unit 48 is electrically connected to the common pad 30for transmitting electrical feedback to the servo control unit 46 whenthe cutting device 44 contacts any one of the contact pads 26 or 26 a.

Although the closed loop backdrilling system 10, and in particular, themultilayer circuit board 16 has been described herein as having only onefeedback layer 22, it should be understood that any number of feedbacklayers 22 can be used within the multilayer circuit board 16. Forexample, the multilayer circuit board 16 can include 2 or more feedbacklayers 22.

The spacing between the contact pads 26 or 26 a and the signal layer 20can vary widely depending on the size and the shape of the cuttingdevice 44 used during the back drilling operation as well as the amountof the stub portion 18 that is desired to be removed. The spacing shouldbe as closed to the signal layer 20 as possible but far enough away sothat the spindle unit 42 of the boring device 40 can be stopped. Forexample, the spacing between the contact pads 26 or 26 a and the signallayer 20 has been found to be suitable between about 0.004 inches and0.001 inches.

To remove the stub portion 18, the measurement unit 48 is connected tothe feedback layer 22, such as by connecting the measurement unit 48 tothe common pad or second plated hole 30. The spindle unit 42 is thenactuated and the servo control unit 46 causes the spindle unit 42 tomove the cutting device 44 toward and into the multilayer circuit board16. The cutting device 44 is moved into the multilayer circuit board 16until electrical feedback is received from the feedback layer 22indicating that a certain depth has been reached. Depending upon theapplication of the closed loop backdrilling system 10, the servo controlunit 46 either 1) causes the spindle unit 42 to retract the cuttingdevice 44, or 2) causes the spindle unit 42 to move the cutting device44 a predetermined distance past the feedback layer 22 before thespindle unit 42 retracts the cutting device 44.

The process set forth above can then be repeated for each and everyplated hole 12 to be backdrilled. The hole(s) formed by the cuttingdevice 44 is/are then filled with an epoxy, or other filler material.

The closed loop back drilling system 10 can also be provided with areporting system 80 for permitting usage of the closed loop backdrilling system 10 to be monitored. The reporting system 80 receivesinput from a portion of the boring device 40, such as the servo-controlunit 46 or the measurement unit 48 to count the number of backdrilledholes in real-time and then provides a report or an invoice periodically(daily, monthly, quarterly, etc.) showing the number of holesbackdrilled on which a license fee is due. The report or invoice can betransmitted electronically to a user of the closed loop backdrillingsystem 10, or a designated collection entity.

The making and using of multilayer circuit boards is well known in theart. Thus, a description of the particular materials, adhesives andother factors utilized in the making of multilayer circuit boards hasnot been discussed in detail herein. However, examples of the variousconstructions and fabrication methods for attaching and forming eachlayer of a multilayer circuit board are disclosed in U.S. Pat. No.5,677,515, the contents of Column 1, Line 8 through Line 55 , Column 2 ,Line 32 through Column 3, Line 7 , and the drawings referenced thereinare hereby incorporated herein by reference.

Shown in FIGS. 5 and 6 is an alternate embodiment of a multilayercircuit board 100 constructed in accordance with the present invention.The multilayer circuit board 100 is constructed similarly to themultilayer circuit board 16 described above, except that the multilayercircuit board 100 is provided with a plated hole 102 having a steppedconfiguration. As will be discussed in more detail below, the steppedconfiguration of the plated hole 102 permits denser connections than thestraight-sided plated hole 12.

The plated hole 102 is provided with a first portion 104 and a secondportion 106. The first portion 104 and the second portion 106 areseparated by a lateral protrusion 108. The second portion 106 has across-sectional area less than the cross-sectional area of the firstportion 104.

The plated hole 102 can be formed in any suitable manner. For example,the plated hole 102 can be formed in one drilling operation with the useof a stepped drill bit and then subsequently plated. Alternatively, theplated hole 102 can be formed in multiple drilling operations with theuse of two cutting devices or drill bits having different diameters andthen subsequently plated.

The multilayer circuit board 100 includes one or more signal layer 120,one or more feedback layer 122, and one or more dielectric layer 124.The feedback layer 122 has a contact pad 126. The dielectric layer 124is positioned between the signal layer 120 and the feedback layer 122.The plated hole 102 is connected to the signal layer 120, and positionedadjacent to the contact pad 126. The contact pad 126 is electricallyisolated from the plated hole 102 by the dielectric layer 124. Ingeneral, the second portion 106 of the plated hole 102 forms the stubportion, which extends a distance away from the signal layer 120 andalso extends a distance away from the contact pad 126 of the feedbacklayer 122.

The plated hole 102 of the multilayer circuit board 100 is thenbackdrilled to remove the stub portion in a similar manner as describedabove, with the exception that the diameter of the cutting device usedin the backdrilling operation can be the same as or even a smallerdiameter than the diameter of the first portion 104 of the plated hole102. This permits a denser population of holes because the diameter ofthe plated hole 102 is not increased during the backdrilling operation.

The concept of embedding a contact pad within a multi-layer circuitboard can be used in accordance with the present invention for at leasttwo other purposes, i.e., depth routing and guide routing.

Depth routing can be used to determine the final level of a the boringdevice 40 penetration depth for clearing out areas on the multi-layercircuit board to receive a component or a connector. In thisapplication, the cutting device 44 of the boring device 40 is moved inthe direction of the multilayer circuit board until the tip of thecutting device 44 contacts the contact pad. The penetration of thecutting device 44 is then stopped or moved a predetermined distancefurther into the multilayer circuit board. The cutting device 44 is thentranslated laterally (x and/or y axis) a predetermined amount to clearout the area for the component or the connector.

Guide routing can be used to guide the lateral translation (x and/or yaxis) of the boring device. That is, once the cutting device 44 reachesthe contact pad, the penetration level of the cutting device 44 isstopped or moved a predetermined distance further into the multilayercircuit board. The boring device 40 begins to move the multilayercircuit board or the cutting device 44 laterally and continues to movelaterally while the cutting device 44 is in contact with the contactpad. When the cutting device 44 loses contact with the contact pad, theboring device 40 retracts or stops.

Shown in FIG. 8 is an alternate embodiment of a closed loop backdrillingsystem constructed in accordance with the present invention. The closedloop backdrilling system 200 is used for depth drilling the alreadyplated hole 12 formed in the workpiece 14 using electrical feedback fromthe workpiece 14 when a certain or predetermined depth is reached. Thefeedback during the drilling process makes it possible to yield muchhigher accuracy independent of the depth of a particular layer withinthe workpiece 14. As discussed above, in one preferred embodiment, theworkpiece 14 is a multilayer printed circuit board 16. Although theworkpiece 14 will be described herein as the multilayer circuit board16, it should be understood that the workpiece 14 can be any apparatus,or device where it is desirable to drill to a certain or predetermineddepth within the apparatus or device.

The multilayer circuit board 16 is provided with at least one or moresignal layer 20, at least one or more feedback layer 22, and at leastone or more dielectric layer 24. The signal layer 20 is connected to theplated hole 12. The feedback layer 22 has a contact pad 202, which issimilar in construction and function as the contact pads 26 or 26 adiscussed above, except that the contact pad 202 is connected to theplated hole 12 to permit electrical communication therebetween. Thedielectric layer 24 is positioned between the signal layer 20 and thecontact pad 202 of the feedback layer 22 so as to electrically isolatethe contact pad 202 from the signal layer 20. As will be discussed inmore detail below, the contact pad 202 is contacted during themanufacturing process of the multilayer circuit board 16 to break theelectrical connection between the plated hole 12 and the contact pad202.

The closed loop backdrilling system 200 is provided with the boringdevice 40. The boring device 40 is provided with the spindle unit 42supporting the cutting device 44, and the servo control unit 46 for theZ-axis motor control.

The closed loop backdrilling system 200 is also provided with ameasurement unit 204. The measurement unit 204 communicates with a) theplated hole 12 and b) the feedback layer 22 for determining loss ofelectrical communication between the plated hole 12 and the feedbacklayer 22. As shown in FIG. 8, the measurement unit 204 is provided incircuit with a resistor R and a power supply P. The measurement unit 204can, for example, be an ammeter for measuring either AC or DC electricalcurrent.

In use, the spindle unit 42 of the boring device 40 is operated to movethe cutting device 44 a predetermined amount in a direction toward orinto the plated hole l2. The spindle unit 42 is then operated to retractthe cutting device 44 a predetermined amount in a direction away fromthe plated hole 12 so that the conductivity of the cuffing device 44will not interfere with the reading of the measurement unit 204. Theservo control unit 46 then monitors the measurement unit 204 todetermine whether the feedback layer 22 is still connected to the platedhole 12. If so, the spindle unit 42 is then operated to move the cuttingdevice 44 in the direction toward the plated hole 12 to remove more ofthe stub portion 18 of the plated hole 12. The spindle unit 42 is thenoperated to retract the cutting device 44 away from the plated hole 12and the servo control unit 46 again monitors the measurement unit 204 todetermine whether the feedback layer 22 is connected to the plated hole12. This process is repeated until the measurement unit 204 determinesthat the plated hole 12 has been disconnected from the feedback layer22.

Shown inFIG. 9 is another alternate embodiment of a closed loopbackdrilling system constructed in accordance with the presentinvention. The closed loop backdrilling system 300 is used for depthdrilling the already plated hole 12 formed in the workpiece 14 usingelectrical feedback from the workpiece 14 when a certain orpredetermined depth is reached. The feedback during the drilling processmakes it possible to yield much higher accuracy independent of the depthof a particular layer within the workpiece 14. As discussed above, inone preferred embodiment, the workpiece 14 is the multilayer printedcircuit board 16. Although the workpiece 14 will be described herein asthe multilayer circuit board 16, it should be understood that theworkpiece 14 can be any apparatus, or device where it is desirable todrill to a certain or predetermined depth within the apparatus ordevice.

The multilayer circuit board 16 is provided with at least one or moresignal layer 20, and at least one or more dielectric layer 24. Thesignal layer 20 is connected to the plated hole 12.

The closed loop backdrilling system 300 is provided with the boringdevice 40. The boring device 40 is provided with the spindle unit 42supporting the cutting device 44, and the servo control unit 46 for theZ-axis motor control.

The closed loop backdrilling system 300 is also provided with a signalgenerator and measurement unit 304. The signal generator and measurementunit 304 communicates with a) the plated hole 12 and b) the signal layer20 for providing a signal to the plated hole 12 and then determining thereflection or interference of the signal caused by the stub portion 18.The signal generator and measurement unit 304 can determine anyappropriate physical property indicative of the existence or amount ofthe stub portion 18, such as the signal to noise ratio.

In use, the spindle unit 42 of the boring device 40 is operated to movethe cutting device 44 a predetermined amount in a direction toward orinto the plated hole 12. The spindle unit 42 is then operated to retractthe cutting device 44 a predetermined amount away from the plated hole12 so that the conductivity of the cutting device 44 will not interferewith the reading of the signal generator and measurement unit 304. Theservo control unit 46 then monitors the signal generator and measurementunit 304 to determine the relatedness between the physical propertybeing measured and a predetermined amount. For example, when the signalto noise ratio is being monitored, a determination can be made (eitherby the servo control unit 42 or the signal generator and measurementunit 304) whether the signal to noise ratio is above a predeterminedamount indicative of removal of a sufficient amount of the stub portion18. If not, the spindle unit 42 is then operated to move the cuttingdevice 44 a predetermined amount in the direction toward the plated hole12 to remove more of the stub portion 18 of the plated hole 12. Thespindle unit 42 is then operated to retract the cutting device 44 awayfrom the plated hole 12 and the servo control unit 46 again monitors thesignal generator and measurement unit 304 to determine whether enough ofthe stub portion 18 has been removed. This process is repeated until thesignals output by the signal generator and measurement unit 304 indicatethat a sufficient amount of the stub portion 18 has been removed.

The closed backdrilling systems 200 and 300 can be operated eithermanually or automatically. In other words, the measurements made by themeasurement unit 204 or the signal generator and measurement unit 304can be made manually by an individual, who is also controlling the servocontrol unit 204. Alternatively, the measurements made by themeasurement unit 204 or the signal generator and measurement unit 304can be automated such that the retractions and advancements of thespindle unit 42 are controlled by a software program typically operatingin or controlling the servo control unit 46.

Changes may be made in the construction and the operation of the variouscomponents, elements and assemblies described herein or in the steps orthe sequence of steps of the methods described herein without departingfrom the spirit and scope of the invention as defined in the followingclaims.

1. A workpiece having a stub portion removed from a plated hole whereinthe amount of the stub portion removed is determined from electricalfeedback received from within the workpiece, the workpiece produced by amethod comprising the steps of: providing the workpiece with a feedbacklayer; boring a hole into the workpiece with a boring device to removeat least a portion of a plated hole until the boring device engages thefeedback layer; and receiving an electrical feedback signal from theboring device indicating engagement with the feedback layer.
 2. Theworkpiece of claim 1, wherein the step of boring is defined further asthe steps of moving the a cutting device of the boring device into theplated hole to remove a portion of the plated hole, stopping themovement of the cutting device and restarting the movement of thecutting device into the plated hole to remove more of the plated hole,and wherein the step of receiving feedback is defined further asreceiving feedback responsive to the cuffing device engaging thefeedback layer.
 3. The workpiece of claim 1, wherein the feedback layeris a ground layer.
 4. The workpiece of claim 1, wherein the feedbacklayer is a power layer.
 5. The workpiece of claim 1, wherein thefeedback layer is a signal layer.
 6. The workpiece of claim 1, whereinthe feedback layer is a separate network.
 7. A multilayer circuit board,comprising: a signal layer; a feedback layer having a contact pad; adielectric layer positioned between the signal layer and the feedbacklayer; and a plated hole connected to the signal layer, and positionedadjacent to the contact pad, the contact pad being electrically isolatedfrom the plated hole, a portion of the plated hole forming a stubportion, which extends a distance away from the signal layer and extendsa distance away from the contact pad of the feedback layer.
 8. Themultilayer circuit board of claim 7, wherein the feedback layer is aground layer.
 9. The multilayer circuit board of claim 7, wherein thefeedback layer is a power layer.
 10. The multilayer circuit board ofclaim 7, wherein the feedback layer is a power layer.
 11. The multilayercircuit board of claim 7, wherein the feedback layer is a separatenetwork.
 12. The multilayer circuit board of claim 7, further comprisinga second plated hole which is electrically connected to the contact pad.13. The multilayer circuit board of claim 7, wherein the contact padsubstantially surrounds the plated hole.
 14. A method for backdrilling astub portion of a plated hole in a multilayer circuit board, comprisingthe steps of: moving a cutting device in a direction toward the platedhole such that the cutting device removes a portion of the platingadjacent the stub portion and disconnects a feedback layer from theplated hole; and determining whether the feedback layer is disconnectedfrom the plated hole.
 15. The method of claim 14, further comprising thestep of retracting the cutting device away from the plated hole so thatthe conductivity of the cutting device will not interfere with thedetermination of whether the feedback layer is disconnected from theplated hole.